arXiv subjects

Antonio Racioppi

Publications and source records attributed to Antonio Racioppi.

At least 19 recordsLinked to original sources

arXiv2026

Inflation with Nieh-Yan-like terms in metric-affine gravity

We study single-field slow-roll inflation in metric-affine gravity with a scalar field non-minimally coupled to the non-Riemannian Ricci scalar and to the divergences of the torsion and nonmetricity vectors, a structure that generalizes the well-known Nieh-Yan term. By imposing projective coherence of the matter sector and solving the connection field equations, we integrate out torsion and nonmetricity and obtain an equivalent Einstein-frame formulation in which the metric-affine couplings are encoded in a modified kinetic function and potential. For the choice of coupling functions $\mathcal{A}(ϕ) = M_P^2 + ξϕ^2$ to the non-Riemannian Ricci scalar, $\mathcal{C}_i(ϕ) = ξ_i ϕ$ to the Nieh-Yan-like terms and a monomial Jordan-frame potential $\mathcal{V} \propto ϕ^k$, we show that in the limit of a large positive effective Nieh-Yan-like coupling $\barξ$ the canonical field satisfies $χ\sim ϕ^2$, the Jordan-frame field values during inflation become sub-Planckian, and the Einstein-frame potential reduces to $U \sim χ^{k/2}$. We compute the slow-roll predictions numerically for quartic and quadratic Jordan-frame potentials and compare them with the current CMB constraints from Planck, BICEP/Keck, ACT, and SPT. We find that intermediate values of $\barξ$ can restore the compatibility of non-minimally coupled Palatini inflation with observations: in the quartic case, the model predicts a tensor-to-scalar ratio within reach of next-generation CMB experiments for $\barξ\lesssim10^4$, while in the quadratic case the coupling cures the $η$-problem arising for $ξ\gtrsim 10^{-2}$ and yields viable predictions for $10^{-2}\lesssim\barξ\lesssim 10^2$. In the negative $\barξ$ regime, the model does not improve upon standard Palatini inflation, though it can still produce distinct, testable predictions.

gr-qc↗

arXiv2026

Quasi-pole inflation in metric-affine gravity

We propose a new mechanism for inflationary model building in the framework of metric-affine gravity. Such a mechanism involves an inflaton non-minimally coupled with the Holst invariant. If the non-minimal coupling function has a zero point and it is very steep at that same point, the corresponding inflaton kinetic function will feature a quasi-pole behaviour, implying a canonically normalized potential featuring an exponential plateau, regardless of the shape of the original inflaton potential. The inflationary predictions in such a region are equivalent to the ones of Starobinsky inflation.

gr-qc↗

arXiv2026

Quasi-pole quintessential inflation in metric-affine gravity

We study quintessential inflation in the framework of metric-affine gravity. It is well known that non-minimal couplings with the Holst invariant can generate a quasi-pole inflationary behaviour resulting in a Starobinsky-like phenomenology. The same quasi-pole behaviour can also be used in order to "flatten" the scalar potential in the Dark Energy era providing a successful framework for quintessential inflation. Agreement with all the observational constraints, reduces the predicted scalar spectral index to a narrow window: $0.966 \lesssim n_s \lesssim 0.967$, making the model highly testable and falsifiable.

gr-qc↗

arXiv2025

Has ACT measured radiative corrections to the tree-level Higgs-like inflation?

Starobinsky inflation and nonminimally coupled (NMC) Higgs inflation have been among the most favored models of the early Universe, as their predictions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ fall comfortably within the constraints set by Planck and BICEP/Keck. However, new results from the Atacama Cosmology Telescope (ACT) suggest a preference for higher values of $n_s$, introducing tension with the simplest realizations of these models. In this work, being agnostic about the nature of the inflaton, we show that incorporating one-loop corrections to a quartic NMC inflationary scenario leads to a shift in the predicted value of $n_s$, which brings NMC inflation into better agreement with ACT observations. The effect is even more significant when the model is formulated in the Palatini approach, where the modified field-space structure naturally enhances deviations from the metric case. These findings highlight the importance of quantum corrections and gravitational degrees of freedom in refining inflationary predictions in light of new data.

astro-ph.CO↗

arXiv2025

Quintessential Inflation in Palatini $F(R,X)$ gravity

Palatini $F(R,X)$ gravity, with $X$ the inflaton kinetic term, proved to be a powerful framework for generating asymptotically flat inflaton potentials. Here we show that a quadratic Palatini $F(R,X)$ restores compatibility with the observational data of the Peebles-Vilenkin quintessential inflation model. Moreover, the same can be achieved with an exponential version of the Peebles-Vilenkin potential if embedded in a Palatini $F(R,X)$ of order higher than two.

gr-qc↗

arXiv2025

$\tildeξ$-attractors in metric-affine gravity

We propose a new class of inflationary attractors in metric-affine gravity. Such class features a non-minimal coupling $\tildeξ\, Ω(ϕ)$ with the Holst invariant $\tilde{\cal R}$ and an inflaton potential proportional to $Ω(ϕ)^2$. The attractor behaviour of the class takes place with two combined strong coupling limits. The first limit is realized at large $\tildeξ$, which makes the theory equivalent to a $\tilde{\cal R}^2$ model. Then, the second limit considers a very small Barbero-Immirzi parameter which leads the inflationary predictions of the $\tilde{\cal R}^2$ model towards the ones of Starobinsky inflation. Because of the analogy with the renown $ξ$-attractors, we label this new class as $\tildeξ$-attractors.

gr-qc↗

arXiv2025

Fractional attractors in light of the latest ACT observations

In light of the latest results from ACT observations we review a class of potentials labeled as fractional attractors, that can originate from Palatini gravity. We show that, for certain choices of the scalar potential $V(ϕ)$, the fractional attractors predict both a spectral index $n_s$ and a tensor-to-scalar ratio $r$ that fall within the $1σ$ region of the combined ACT+Planck data for a wide range of parameters. We also provide a numerical fit for the parameter space of this models in the case of a simple quadratic and quartic fractional potential.

gr-qc↗

arXiv2025

Symmetry-breaking inflation in non-minimal metric-affine gravity

We study symmetry-breaking inflation within the framework of metric-affine gravity. By introducing a non-minimal coupling, $β(ϕ)\tilde{\cal R}$, between the Holst invariant and the inflaton, both small-field and large-field inflationary predictions can be brought into agreement with the latest observational constraints. Remarkably, even for sub-Planckian vacuum expectation values, appropriately chosen values of $β(ϕ)$ enable viable inflation, a scenario previously considered unattainable.

gr-qc↗

arXiv2025

Interpreting DESI 2024 BAO: late-time dynamical dark energy or a local effect?

We perform fits to DESI, CMB and supernova data to understand the physical origin of the DESI hint for dynamical dark energy. We find that the linear parametrization of the equation of state $w$ may guide to misleading interpretations, such as the hint for a phantom Universe, which are not preferred by the data. Instead, physical quintessence models fit the data well. Model-independently, present observations prefer deviations from the constant dark energy, $w=-1$, only at very low redshifts, $z < \mathcal{O}(0.1)$. We find that this result is driven by low-$z$ supernova data. Therefore, either the fundamental properties of our Universe, characterised by the equation of state $w$ and the Hubble parameter $H$, underwent dramatic changes very recently or, alternatively, we do not fully understand the systematics of our local Universe in a radius of about $300\,h^{-1}\rm Mpc$.

astro-ph.CO↗

arXiv2024

Natural Metric-Affine Inflation

We consider here natural inflation in the low energy (two-derivative) metric-affine theory containing only the minimal degrees of freedom in the inflationary sector, i.e. the massless graviton and the pseudo-Nambu-Goldstone boson (PNGB). This theory contains the Ricci-like and parity-odd Holst invariants together with non-minimal couplings between the PNGB and the above-mentioned invariants. The Palatini and Einstein-Cartan realizations of natural inflation are particular cases of our construction. Explicit models of this type featuring non-minimal couplings are shown to emerge from the microscopic dynamics of a QCD-like theory with an either sub-Planckian or trans-Planckian confining scale and that is renormalizable on Minkowski spacetime. Moreover, for these models, we find regions of the parameter space where the inflationary predictions agree with the most recent observations at the $2σ$ level. We find that in order to enter the $1σ$ region it is necessary (and sufficient) to have a finite value of the Barbero-Immirzi parameter and a sizable non-minimal coupling between the inflaton and the Holst invariant (with sign opposite to the Barbero-Immirzi parameter). Indeed, in this case the potential of the canonically normalized inflaton develops a plateau as shown analytically.

hep-ph↗

arXiv2024

Palatini $F(R,X)$: a new framework for inflationary attractors

Palatini $F(R)$ gravity proved to be a powerful tool in order to realize asymptotically flat inflaton potentials. Unfortunately, it also inevitably implies higher-order inflaton kinetic terms in the Einstein frame that might jeopardize the evolution of the system out of the slow-roll regime. We prove that a $F(R+X)$ gravity, where $X$ is the inflaton kinetic term, solves the issue. Moreover, when $F$ is a quadratic function such a choice easily leads to a new class of inflationary attractors, fractional attractors, that generalizes the already well-known polynomial $α$-attractors.

gr-qc↗

arXiv2024

Beyond (and back to) Palatini quadratic gravity and inflation

We study single-field slow-roll inflation embedded in Palatini $F(R)$ gravity where $F(R)$ grows faster than $R^2$. Surprisingly, the consistency of the theory requires the Jordan frame inflaton potential to be unbounded from below. Even more surprisingly, this corresponds to an Einstein frame inflaton potential bounded from below and positive definite. We prove that for all such Palatini $F(R)$'s, there exists a universal strong coupling limit corresponding to a quadratic $F(R)$ with the wrong sign for the linear term and a cosmological constant in the Jordan frame. In such a limit, the tensor-to-scalar ratio $r$ does not depend on the original inflaton potential, while the scalar spectral index $n_s$ does. Unfortunately, the system is ill-defined out of the slow-roll regime. A possible way out is to upgrade to a $F(R,X)$ model, with $X$ the Jordan frame inflaton kinetic term. Such a modification essentially leaves the inflationary predictions unaffected.

gr-qc↗

arXiv2024

Quintessence in the Weyl-Gauss-Bonnet model

Quintessence models have been widely examined in the context of scalar-Gauss-Bonnet gravity, a subclass of Horndeski's theory, and were proposed as viable candidates for Dark Energy. However, the relatively recent observational constraints on the speed of gravitational waves $c_{\textrm{GW}}$ have resulted in many of those models being ruled out because they predict $c_{\textrm{GW}} \neq c$ generally. While these were formulated in the metric formalism of gravity, we put forward a new quintessence model with the scalar-Gauss-Bonnet action but in Weyl geometry, where the connection is not metric compatible. We find the fixed points of the dynamical system under some assumptions and determine their stability via linear analysis. The past evolution of the Universe can be reproduced correctly, but the late Universe constraints on $c_{\textrm{GW}}$ are grossly violated. Moreover, at these later stages tensor modes suffer from the gradient instabilities. We also consider the implications of imposing an additional constraint $c_{\textrm{GW}} = c$, but this does not lead to evolution that is consistent with cosmological observations

gr-qc↗

arXiv2024

Pseudo-Goldstone dark matter in a radiative inverse seesaw scenario

We consider a scale-invariant inverse seesaw model with dynamical breaking of gauge symmetry and lepton number. In some regions of the parameter space, the Majoron - the pseudo-Goldstone of lepton number breaking - is a viable dark matter candidate. The bound on the Majoron decay rate implies a very large dilaton vacuum expectation value, which also results in a suppression of other dark matter couplings. Because of that, the observed dark matter relic abundance can only be matched via the freeze-in mechanism. The scalar field which gives mass to heavy neutrinos can play the role of the inflaton, resulting in a tensor-to-scalar ratio $r \lesssim 0.01$ for metric inflation and $r \lesssim 0.21$ for Palatini gravity.

hep-ph↗

arXiv2023

Gauss-Bonnet Dark Energy and the Speed of Gravitational Waves

Gauss-Bonnet Dark Energy has been a popular model to explain the accelerated expansion of the Universe. Quite generically it also predicts the speed of gravitational waves $c_{GW}$ to be different from the speed of light. This fact alone led some authors to exclude such models in view of the new tight observational constraints on $c_{GW}$. However, the behaviour of $c_{GW}$ depends on the choice of the Gauss-Bonnet (GB) coupling function. It is possible to construct models where $c_{GW}$ is always equal to the speed of light. More generally, $c_{GW}$ is a time dependent function with instances where both speeds coincide. Nevertheless, we observe that the bound on $c_{GW}$ excludes scenarios where the GB term directly affects the expansion of the Universe, even if the constraint on the variation of the coupling function does not appear to be strong. We perform the dynamical systems analysis to see if the expansion of the Universe could be affected indirectly by modulating the behaviour of the scalar field, which modulates the GB coupling. It is shown that either the bounds on $c_{GW}$ are violated by many orders of magnitude, or it might be very difficult to find models that are consistent with other cosmological observations.

astro-ph.CO↗

arXiv2023

Generalized Hilltop Inflation

We study a generalized version of hilltop inflation where the standard hilltop potential has been raised to a power and we allow fractional numbers for both the original hilltop power ($m$) and the overall exponent ($n$). In the parameter space studied, agreement with the latest experimental constraints favors high values for $m$ and low values for $n$. Finally, we also find that in all the configurations studied, the inflationary scale always sits around the grand unification scale.

astro-ph.CO↗

arXiv2023

Primordial black holes and inflation from double-well potentials

We investigate the formation of large peaks in the inflationary curvature power spectrum from double-well potentials. In such scenarios, the initial CMB spectrum is created at large field values. Subsequently, the inflaton will cross one of the minima and will decelerate rapidly as it reaches the local maximum at the origin, either falling back or crossing it. During this final phase, a significant peak in the curvature power spectrum can be generated. Our analysis reveals that this class of models produces more pronounced peaks than the more commonly studied quasi-inflection point scenarios with less tuning for the model parameters. Finally, we construct an explicit theoretically motivated inflationary scenario that is consistent with the latest CMB observations and capable of generating sufficiently large curvature perturbations for primordial black holes.

astro-ph.CO↗

arXiv2022

Slow-roll inflation in Palatini $F(R)$ gravity

We study single field slow-roll inflation in the presence of $F(R)$ gravity in the Palatini formulation. In contrast to metric $F(R)$, when rewritten in terms of an auxiliary field and moved to the Einstein frame, Palatini $F(R)$ does not develop a new dynamical degree of freedom. However, it is not possible to solve analytically the constraint equation of the auxiliary field for a general $F(R)$. We propose a method that allows us to circumvent this issue and compute the inflationary observables. We apply this method to test scenarios of the form $F(R) = R + αR^n$ and find that, as in the previously known $n=2$ case, a large $α$ suppresses the tensor-to-scalar ratio $r$. We also find that models with $F(R)$ increasing faster than $R^2$ for large $R$ suffer from numerous problems.

gr-qc↗